# Size Tankless Water Heater Calculator

Size tankless water heater calculator returns required GPM and BTU by bathrooms, family size, and climate. Tankless water heater size chart, gas and electric feasibility check.

## What this calculates

Sizing a tankless water heater is different from sizing a tank unit: you size by peak simultaneous flow (GPM) and required temperature rise, not by stored gallons. This size tankless water heater calculator combines household simultaneous-fixture demand with your local inlet water temperature to return the exact GPM and BTU input your unit needs, and checks whether a gas or electric tankless can meet the spec.

## Inputs

- **Number of Bathrooms** — options: 1 bathroom (single or couple), 2 bathrooms, 3 bathrooms, 4 bathrooms, 5+ bathrooms (large home) — More bathrooms means more simultaneous hot water demand.
- **Household Size** — options: 1 person, 2 people, 3 people, 4 people (family of 4), 5 people, 6+ people (large family) — For a family of 4 with 2 bathrooms, 7-8 GPM is typical.
- **Climate Zone (Inlet Water Temp)** — options: Warm (FL, TX, AZ) - 70°F inlet, Moderate (CA, Mid-Atlantic) - 55°F inlet, Cold (Midwest, NE, Northern) - 45°F inlet, Very cold (Alaska, Northern Plains) - 35°F inlet — Cold inlet water requires bigger BTU or GPM to reach 120°F output.
- **Desired Hot Water Temperature** (°F) — min 105, max 140 — 120°F is standard. Higher temps need more BTU per gallon.
- **Preferred Fuel Type** — options: Natural gas / propane (highest GPM), Electric (simpler install, lower max GPM), Either (show recommendation)
- **Hot Water Usage Pattern** — options: Low (light usage, short showers), Normal (typical household), High (long showers, simultaneous fixtures common)

## Outputs

- **Required GPM** — Minimum simultaneous flow rate your tankless must deliver.
- **Required Temperature Rise** (°F) — Output temp minus inlet temp.
- **Required BTU Input (gas)** (BTU/hr) — GPM x temp rise x 500.
- **Recommended Unit Class** — formatted as text — Best-fit unit type for your spec.
- **Gas Tankless Feasible** — formatted as text
- **Electric Tankless Feasible** — formatted as text

## Details

## How to Size a Tankless Water Heater

Tankless sizing is two variables: GPM (gallons per minute) and temperature rise.

### GPM: Simultaneous Flow Demand

Add up the flow rates of fixtures you might run at the same time:

| Fixture | Flow Rate |
|---------|-----------|
| Low-flow shower | 2.0 GPM |
| Standard shower | 2.5 GPM |
| Kitchen sink | 1.5 GPM |
| Bathroom sink | 1.0 GPM |
| Dishwasher | 1.5 GPM |
| Clothes washer | 2.0 GPM |

Example: 2 showers + kitchen sink = 2(2.0) + 1.5 = 5.5 GPM

### Temperature Rise

Temperature rise = desired output (120F standard) minus inlet water temperature. Inlet water temp varies by climate:

| Region | Inlet Temp | Rise to 120F |
|--------|-----------|--------------|
| Warm (FL, TX, AZ) | 70F | 50F |
| Moderate (CA, Mid-Atlantic) | 55F | 65F |
| Cold (Midwest, NE) | 45F | 75F |
| Very cold (Northern Plains) | 35F | 85F |

### BTU Input Required

**BTU/hr = GPM x Temp Rise x 500**

Example: 5.5 GPM x 65F rise x 500 = 178,750 BTU/hr. That needs a premium residential gas tankless like a Rinnai RU180 or RU199, or a Navien NPE-240A2.

## Tankless Water Heater Size Chart

The tankless water heater size chart below maps household size to GPM and unit class:

| Household | Bathrooms | GPM Needed | Typical Unit |
|-----------|-----------|------------|--------------|
| 1-2 people | 1 bath | 3-5 GPM | Rinnai V65, Rheem RTG-64 |
| Family of 4 | 2 baths | 7-8 GPM | Rinnai V75, Rheem RTGH-84 |
| Family of 5-6 | 3 baths | 9-10 GPM | Rinnai RUC98, Navien NPE-240A2 |
| 6+ people | 4+ baths | 10-12 GPM | Cascaded or commercial unit |

## Tankless Water Heater Size for Family of 4

Tankless water heater size for family of 4 with 2 bathrooms is typically 7-8 GPM in moderate climates. In cold climates, the same family needs a 9-10 GPM unit because the temperature rise is higher. A 7 gpm tankless water heater covers most families of 4 in warm-to-moderate climates; step up to 10 gpm tankless water heater in cold climates.

## 10 GPM Tankless Water Heater

A 10 gpm tankless water heater handles 3-4 simultaneous fixtures in warm climates, 2-3 simultaneous fixtures in cold climates. Popular 10 GPM units: Rinnai RUC98i (199k BTU gas), Navien NPE-240A2 (199k BTU gas), Stiebel Eltron Tempra 36 (36 kW electric, 4 circuits).

## 10 GPM Tankless Water Heater Electric

A 10 gpm tankless water heater electric is only practical in warm climates (70F inlet). Units like the Stiebel Eltron Tempra 36 or EcoSmart ECO 36 deliver 10 GPM at a 35-40F temperature rise (warm climates), but drop to 6-7 GPM at the 65-75F rise cold climates require. For 10 GPM in cold climates, you need gas.

## 7 GPM Tankless Water Heater

A 7 gpm tankless water heater suits a family of 4 with 2 bathrooms in warm-to-moderate climates (50-65F rise). Gas models: Rinnai V75iN (180k BTU), Takagi T-K4 (190k BTU). Electric: Stiebel Eltron Tempra 29 (29 kW, 3 x 40A circuits).

## 7 GPM Tankless Water Heater Electric

A 7 gpm tankless water heater electric is borderline: the Tempra 29 or EcoSmart ECO 29 delivers 7 GPM at 50F rise (warm climate) but drops to 4-5 GPM at 75F rise (cold). Confirm your electrical panel can support 3 x 40A circuits (120A of dedicated load, 200A service minimum).

## 5 GPM Tankless Water Heater

A 5 gpm tankless water heater covers 1-bathroom homes or couples with 2 bathrooms. In gas: Rheem RTG-64 (140k BTU), Rinnai V53. In electric: Stiebel Eltron Tempra 24 (24 kW, 2 x 40A circuits). Works in all climates when sized correctly.

## What Size Electric Tankless Water Heater Do I Need

What size electric tankless water heater do I need depends on climate. The electric tankless water heater size chart:

| Family / Bathrooms | Warm Climate (70F inlet) | Cold Climate (45F inlet) |
|--------------------|-------------------------|-------------------------|
| 1 person / 1 bath | 10-12 kW (3-4 GPM) | 18-24 kW (3 GPM) |
| Couple / 1 bath | 13-18 kW (4-5 GPM) | 24-29 kW (4 GPM) |
| Family of 4 / 2 bath | 24-29 kW (6-7 GPM) | 29-36 kW (5 GPM) |
| Family of 5 / 3 bath | 29-36 kW (8-10 GPM) | Requires gas |

Electric tankless above 29 kW requires 200A service panel and 3-4 dedicated 40A breakers. In cold climates, gas is almost always better for whole-house tankless.

## Rinnai Tankless Water Heater Models

Popular Rinnai tankless water heater models:

- **Rinnai V53DeN:** 5.3 GPM, 120k BTU, entry level
- **Rinnai V65eN:** 6.5 GPM, 150k BTU, couple or small family
- **Rinnai V75iN:** 7.5 GPM, 180k BTU, family of 4 moderate climate
- **Rinnai V94eN:** 9.4 GPM, 199k BTU, family of 4 cold climate
- **Rinnai RUC98iN:** 9.8 GPM, 199k BTU, condensing (higher efficiency)
- **Rinnai RUCS65iN:** 6.5 GPM, 160k BTU, ultra-condensing

## Electric Tankless Water Heater Size Chart

An electric tankless water heater size chart keyed to kW rating:

| kW | Max GPM (warm) | Max GPM (moderate) | Max GPM (cold) |
|----|----------------|-------------------|-----------------|
| 13 kW | 3.5 | 2.5 | 1.8 |
| 18 kW | 5.0 | 3.5 | 2.5 |
| 24 kW | 6.5 | 5.0 | 3.5 |
| 27 kW | 7.5 | 5.5 | 4.0 |
| 29 kW | 8.0 | 6.0 | 4.5 |
| 36 kW | 10.0 | 7.5 | 5.5 |

## Frequently Asked Questions

**Q: What size tankless water heater for family of 4?**

A: Tankless water heater size for family of 4 with 2 bathrooms is 7-8 GPM in warm to moderate climates, 9-10 GPM in cold climates. A 7 gpm tankless water heater (Rinnai V75iN, Rheem RTGH-84) covers most families of 4 in warm regions. For cold climates (45F inlet), step up to a 10 gpm tankless water heater like the Rinnai RUC98iN.

**Q: What does a 10 gpm tankless water heater cover?**

A: A 10 gpm tankless water heater covers 3-4 simultaneous fixtures in warm climates (70F inlet), or 2-3 simultaneous fixtures in cold climates (45F inlet). Popular 10 GPM models: Rinnai RUC98iN (199k BTU gas), Navien NPE-240A2 (199k BTU gas), Stiebel Eltron Tempra 36 (36 kW electric). A 10 gpm tankless water heater electric works only in warm climates at rated capacity.

**Q: What size electric tankless water heater do I need?**

A: What size electric tankless water heater do I need depends on GPM and temp rise. A couple / 1 bathroom: 18-24 kW (4-5 GPM). Family of 4 / 2 bathrooms: 24-29 kW warm climate, 29-36 kW cold. Family of 5+ / 3+ bathrooms: 36 kW minimum, usually requires gas in cold climates. Check the electric tankless water heater size chart: units over 29 kW need 200A service and 3-4 dedicated 40A circuits.

**Q: How do I read a tankless water heater size chart?**

A: A tankless water heater size chart lists GPM output at different temperature rises. Manufacturers publish max GPM at 35F rise (warm climate) and 77F rise (cold climate). A unit rated 10 GPM at 35F rise only delivers about 6 GPM at 77F rise. Always size by your actual temperature rise (desired output minus inlet temp), not by marketing max GPM.

**Q: What are the main Rinnai tankless water heater models?**

A: Main Rinnai tankless water heater models include: V53DeN (5.3 GPM, 120k BTU), V65eN (6.5 GPM, 150k BTU), V75iN (7.5 GPM, 180k BTU), V94eN (9.4 GPM, 199k BTU), RUC98iN (9.8 GPM, 199k BTU condensing, 0.96 UEF), and the RUR commercial series. The V series is non-condensing (82% efficient); RU and RUC series are condensing (94-96% efficient).

**Q: Is a 5 GPM tankless water heater enough?**

A: A 5 gpm tankless water heater is enough for a 1-bathroom home or a couple with 2 bathrooms who don't run multiple showers simultaneously. It is not enough for a family of 4 with 2 or more bathrooms. Run the size tankless water heater calculator with your actual climate zone - cold climates need 20-40% more GPM than warm climates for the same household.

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Source: https://vastcalc.com/calculators/construction/tankless-water-heater
Category: Construction
Last updated: 2026-04-08